The present invention relates to a method and an apparatus for forming an image from a sequence of images and, more particularly, but not exclusively to a method and a mobile device for generating an image from a sequence of images using motion detection techniques.
In recent years, the demand for high performance compact digital imaging devices has increased. Such imaging devices convert an image of an intercepted scene into electronic signals by using an image sensor, such as a charge-coupled device (CCD) based sensor or a complementary metal-oxide semiconductor (CMOS) based sensor. In particular, the demand for high performance compact digital imaging devices, which are designed to be mounted in a compact device, such as a mobile phone, and have image sensors that have large numbers of pixels, for example more than two million pixels, is increasing. Such a demand is an outcome of the prevalence of mobile devices that incorporate digital cameras, such as laptops, webcams, mobile phones, personal digital assistants (PDAs) and the like.
One of the characteristics that determine the quality of an image is the exposure. In digital photography, exposure is the total amount of light allowed to fall on the image sensor during the process of taking an image. Such a total is a combination of an exposure period, which is the length of time during which light is allowed to fall on the image sensor, and the level of illumination received by the image sensor during the exposure period. If the image sensor is exposed for a longer period, the level of illumination can be reduced and vice versa.
As compact digital imaging devices are limited in size, they usually have relatively miniature lens apertures that admit only a limited amount of light and therefore provide a low level of illumination. As such, a longer exposure period is required in order to provide images with high quality and/or higher level of illumination.
However, long exposure periods may cause visual effects, such as a motion blur, that may degrade the quality of the image. As the camera integrates light coming from the scene for the entire length of the exposure time to generate a frame such as a bitmap image, a motion during this exposure time of moving objects may produce a noted blur along their trajectory. The noted blur may often result in distorted or unrecognizable object shapes. The faster the movement of the object, the stronger the blurring effect is found to be.
A number of system and devices have been developed to confront with the visual effects and/or the quality degradation caused by a low level of illumination. For example, U.S. Pat. No. 6,987,530, published on Jan. 13, 2003 discloses a method and apparatus for detecting and reducing motion blur caused by the movement between a camera and an object including calculation of first and second figures of merit associated with two substantially orthogonal directions and comparison of the figures of merit and adjustment of the magnitude of the amplitude of spatial frequencies of the image in response to the comparison.
Another example is described in U.S. Pat. No. 6,100,927, published on Aug. 8, 2000 that discloses a photographing apparatus, in which image information in a range wider than an ordinary range is converted into an electric signal by an image pickup and stored into an image memory, blurring is corrected by moving a reading-out area from the image memory in accordance with the blurring, and when the blurring correction is not performed, the image stored in the image memory is reduced to a predetermined size at a reduction ratio according to a zoom ratio by a zoom switch.